Block copolymers have been prepared by coupling a living polymer with a multifunctional coupling agent reactive with the polymer-alkali metal moieties, and thereafter shortstopping, if necessary, the reaction with a material which inactivates any of the alkali metal-carbon bonds remaining. In present processes, in order to produce multiblock copolymers, a single monomer is polymerized to form a first block A, dissimilar monomer is copolymerized thereonto to produce a block copolymer AB-structure, and the resulting AB-diblock material then can be terminated, or can be coupled with a dipolyfunctional reagent to produce a coupled or branched polymer.
If both monomers are charged simultaneously, and typically in hydrocarbon media with lithium-based initiators, a highly reactive monomer such as butadiene as opposed to styrene tends to polymerize more rapidly. The result is substantially pure polybutadiene tapering with increasing amounts of copolymerized styrene into a substantially polystyrene block, which then can be coupled.
In sequential monomer addition followed by coupling, as well as in mixed monomer polymerization followed by coupling, coupling efficiencies may not be as high as desired due to the extent of conversion that must occur before the coupling agent is employed, since the coupling agent must "find" the active polymer-alkali metal, e.g., polymer-lithium, entities in order to most efficiently perform its coupling purpose.